[sh/8u] Root processing related backports

[Zhengyu Gu]

> 
> JDK-8189360 is not backported. Look in JNIHandleBlock::weak_oops_do:
> 
>    /*
>     * JVMTI data structures may also contain weak oops.  The iteration of them
>     * is placed here so that we don't need to add it to each of the collectors.
>     */
>    JvmtiExport::weak_oops_do(is_alive, f);

Looked at wrong place. Okay, recovered to original form and added comments:

   // When we're done marking everything, we process weak references.
   // It is not obvious, but reference processing actually calls
   // JNIHandle::weak_oops_do() to cleanup JNI and JVMTI weak oops.
   if (_heap->process_references()) {
     weak_refs_work(full_gc);
   } else {
     weak_roots_work();
   }

Updated: 
http://cr.openjdk.java.net/~zgu/shenandoah/sh-8u-root-processing/webrev.03/

-Zhengyu

> 
> So it would do JvmtiExport::weak_oops_do it during JNIHandle::weak_oops_do. Please make sure we
> never ever touch JvmtiExport::weak_oops_do if JNIHandle::weak_oops_do possibly runs in another worker.
> 
> I would check myself tomorrow.
>